I get the impression that, because of the two broken seats in first, the woman who had been moved to 9D (and subsequently paid to sit in 32D) had been in first (likely an upgrade) and been downgraded due to the seat malfunction. AA then, it seems to me, tried to placate her somewhat with the OP's exit row seat. If this is the case, it makes sense to me, although the voucher was not necessary. It did help smooth things, though, I'm sure.

Cheers.